In the field of superconductive ceramic materials, large advances have been made since the 1970's, and in particular, since 1985. Whereas a decade or so ago superconductivity was observed only at or near the liquid helium temperature, of 1.degree. Kelvin (K), recent advances have raised the superconducting temperature to nearly 100.degree. K. These advances have now brought superconducting ceramics within the temperature range of liquid nitrogen which results in considerable cost savings. While many factors have contributed to expanding the temperature range of superconductive materials, one of the important requirements is the use of materials in the manufacture of superconducting ceramics which will not leave undesirable impurities in the final product. Among the more ubiquitous impurities which might find their way into finished superconducting ceramics are carbon, aluminum, silicon and chlorine (as chloride ion).
The earliest of the higher temperature superconducting ceramic materials were quaternary ceramics composed of yttrium or a lanthanide series rare earth metal, an alkaline earth metal such as barium or strontium, copper and oxygen. More recent discoveries have brought forth five component superconducting ceramics such as the thallium-calcium-barium-copper-oxygen series of materials. While the mechanism by which these ceramic materials are superconducting is not fully understood, there is some speculation that the electrons flowing in these superconducting ceramics travel through "tunnels" formed by alternating layers of metal and oxide ions. While the crystal structure of these ceramics is complex and not fully determined, it is known that they are layered structures sensitive to moisture, heat and pressure. It is further known that distortions in the crystal structure can greatly decrease or completely destroy the superconducting properties of the ceramic material. These distortions can be created in the crystal by the inclusion of larger or smaller ions within the crystal lattice. It is easier to envision such a distortion using a larger ion.
Imagine a large box full of baseballs neatly stacked in layers, one on top of the other and held in place by the walls of the box. The void space between the baseballs would be the "tunnels" through which "electrons" would flow. However, if a basketball were placed in the center of the box, the layered order of the baseballs would be locally destroyed and the flow of "electrons" blocked. Likewise, small marbles placed in void spaces formed by the baseball layers would block the flow of "electrons." For these reasons, impurities such as carbon, aluminum, silicon and chloride ion are not desirable in a superconducting ceramic.
The reported superconducting ceramics have been prepared by thermal conversion using various inorganic salts such as nitrate, carbonate or metal-organics such as alkoxide starting materials. Care in preparation of these starting materials will eliminate silicon and other cations as impurities. Thermal conversion at 800.degree.-1000.degree. C. in an oxygen rich atmosphere causes the carbon present in the starting materials to be oxidized to carbon dioxide gas which volatilizes from the ceramic product, thereby removing carbon as an impurity. Nitrate decomposes to volatile nitrogen oxides and likewise escapes from the ceramics. While nitrates do not leave impurities in the ceramic product, their use creates air pollution control problems in a production scale-up. The removal of chloride, however, cannot so readily be accomplished.
While chloride may not, at first glance, seem to be a problem in the production of superconducting materials, in actual fact chloride salts are frequently used as the base materials from which the carbonates, nitrates and alkoxides are prepared. During these preparations, trace amounts of chloride ion trail along with the carbonate, nitrate or alkoxide product. Removal of the trace chloride may entail numerous purification steps and be prohibitively expensive. Among the most difficult of the raw materials to purify are the copper (II) alkoxides used in the preparation of superconducting ceramics. The terms copper (II) and cupric refer to copper in the +2 valence state.
The most common method of preparing a copper (II) alkoxide has been the reaction of an alcoholic solution of anhydrous cupric chloride with an alcoholic solution of an alkali metal alkoxide prepared in situ by reacting an alkali metal with an excess of an alcohol. While any of the alkali metals may be used, the preferred metal is lithium for safety reasons. The alcohol is typically one of C.sub.1 to C.sub.4 alcohols, although others may be used when desirable. A typical reaction may be that between lithium metal and anhydrous methanol to produce a methanolic solution of lithium methoxide. This lithium methoxide solution is filtered to remove any precipitate that may have formed, and the filtered solution is added to a solution of anhydrous cupric chloride in anhydrous methanol. The lithium methoxide and cupric chloride react to form copper (II) methoxide, which precipitates, and lithium chloride which is soluble in methanol. The copper methoxide is collected by filtration and washed with several methanol washes.
While the reaction of cupric chloride and lithium methoxide seemingly separates the copper (II) methoxide from lithium chloride; in fact, some of lithium chloride and also cupric chloride may become entrained in the precipitate. While washing the precipitated copper (II) methoxide with methanol may remove some of the chloride compounds, sufficient chloride may remain to adversely affect superconductivity in the final ceramic product. It would be preferable that the copper (II) methoxide remain in solution and that the chloride-containing compounds precipitate. Being aware of the difficulties encountered in the preparation of copper (II) alkoxides, this invention presents a new method for the preparation of copper (II) alkoxides uncontaminated by chloride-containing materials. Specifically, the invention relates to the preparation of ultra pure copper (II) alkoxides wherein the copper (II) alkoxide is free of anions as well as any cations which may adversely affect the superconducting properties of copper containing superconducting ceramics.